Lithography is a process for producing patterns of two dimensional shapes, consisting of drawing primitives such as lines and pixels within a layer of material, such as, for example, a resist coated on a semiconductor device. Conventional photolithography (also called optical lithography) is running into problems as the feature size is reduced, e.g., below 45 nm. These problems arise from fundamental issues such as sources for the low wavelength of light, photoresist collapse, lens system quality for low wavelength light and masks cost. To overcome these issues, alternative approaches are required.
Examples of such alternative approaches are known in the field of the so-called nanolithography, which can be seen as high resolution patterning of surfaces. Nano lithography refers to fabrication techniques of nanometer-scale structures, comprising patterns having one dimension typically sizing up to about 100 nm (hence partly overlapping with photolithography). Beyond the conventional photolithography, they further include such techniques as charged-particle lithography (ion- or electron-beams), nanoimprint lithography and its variants, and SPL (for patterning at the nanometer-scale). SPL is for instance described in detail in Chemical Reviews, 1997, Volume 97 pages 1195 to 1230, “Nanometer-scale Surface Modification Using the Scanning Probe microscope: Progress since 1991”, Nyffenegger et al. and the references cited therein, see also Garcia, R.; Martinez, R. V. & Martinez, J., Nano-chemistry and scanning probe nanolithographies, Chem. Soc. Rev., Royal Society of Chemistry, 2006, 35, 29-38.
In general, SPL is used to describe lithographic methods wherein a probe tip is moved across a surface to form a pattern. Scanning probe lithography makes use of scanning probe microscopy (SPM) techniques. SPM techniques rely on scanning a probe, e.g., a sharp tip, in close proximity with a sample surface whilst controlling interactions between the probe and the surface. A confirming image of the sample surface can afterwards be obtained, typically using the same scanning probe in a raster scan of the sample. In the raster scan the probe-surface interaction is recorded as a function of position and images are produced as a two-dimensional grid of data points.
The lateral resolution achieved with SPM varies with the underlying technique: atomic resolution can be achieved in some cases. Use can be made of piezoelectric actuators to execute scanning motions with a precision and accuracy, at any desired length scale up to better than the atomic scale. The two main types of SPM are the scanning tunneling microscopy (STM) and the atomic force microscopy (AFM). In the following, acronyms STM/AFM may refer to either the microscopy technique or to the microscope itself.
In particular, the AFM is a device in which the topography of a sample is modified or sensed by a probe mounted on the end of a cantilever. As the sample is scanned, interactions between the probe and the sample surface cause pivotal deflection of the cantilever. The topography of the sample may thus be determined by detecting this deflection of the probe. Yet, by controlling the deflection of the cantilever or the physical properties of the probe, the surface topography may be modified to produce a pattern on the sample.
Following this idea, in a SPL device, a probe is raster scanned across a functional surface and brought to locally interact with the functional material. By this interaction, material on the surface is removed or changed. In this respect, one may distinguish amongst:
constructive probe lithography, where patterning is carried out by transferring chemical species to the surface; and
destructive probe lithography, which consists of physically and/or chemically deforming a substrate's surface by providing energy (mechanical, thermal, photonic, ionic, electronic, X-ray, etc.).
SPL is accordingly a suitable technique for nanolithography.
High resolution patterning of surfaces is relevant to several areas of technology, such as alternatives to optical lithography, patterning for rapid prototyping, direct functionalization of surfaces, mask production for optical and imprint lithography, and data storage.
In particular, lithography can be used for the fabrication of microelectronic devices. In this case, next to conventional lithography, electron-beam (or e-beam) and probe-based lithography are mostly in use.
For high resolution optical mask and nano-imprint master fabrication, e-beam lithography is nowadays a standard technology. However, when approaching high resolutions, writing times for e-beam mask/master fabrication increase unfavorably.
As a possible alternative, the use of probes for such patterning is still under development. At high resolution (<32 nm), the speed of single e-beam and single probe structuring converges.
In the case of data storage, various approaches have been proposed to make use of probes for storage in the archival regime. However, a main challenge that remains is to achieve long bit-retention. Using thermomechanical indentation allows for instance to achieve satisfactory endurance and retention of data. A thermomechanical approach, however, produces indentations that are inherently under mechanical stress. Therefore it is difficult to obtain retention times in excess of ten years, as usually needed in the archival domain.